My wife is philipino, she has a green card now for almost 10 years we booked a flight to the Philippines dec 1 2015 and return flight may 26 2016. Will we be OK with ? Will her green card be legal?

When does it expire?

I think its more about how long she stays out. New regulations limit how long a green card holder can be outside the USA every year (because people were getting them then not using them for very long periods)... and she is dangerously close to the 6 month point... which is when they will revoke the green card for failure to maintain presence if they exceed it.

1. If it is a real green card and legal
2. If it does not expire while out of the country
3. There is nothing else that you did not mention that would have caused immigration issues.

The green card allows for unlimited entry and exit from the US. (assuming this is the US) So what are the dates on her card?

An extended visit that doesn't exceed 180 days should be fine if she has been an LPR and in the US for the last 10 years. Typically you will have tax returns, bank statements, employment records, housing, drivers license etc. to show your ties to the US.